﻿#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
    vector<int> ret;
    vector<int> index; // 记录 nums 中当前元素的原始下标
    int tmpNums[500010];
    int tmpIndex[500010];

public:
    vector<int> countSmaller(vector<int>& nums) {
        int n = nums.size();
        ret.resize(n);
        index.resize(n);
        // 初始化⼀下 index 数组
        for (int i = 0; i < n; i++)
            index[i] = i;
        mergeSort(nums, 0, nums.size() - 1);
        return ret;
    }
    void mergeSort(vector<int>& nums, int left, int right) {
        if (left >= right)
            return;
        // 1. 根据中间元素，划分区间
        int mid = (left + right) >> 1;
        // [left, mid] [mid + 1, right]
        // 2. 先处理左右两部分
        mergeSort(nums, left, mid);
        mergeSort(nums, mid + 1, right);
        // 3. 处理⼀左⼀右的情况
        int cur1 = left, cur2 = mid + 1, i = 0;
        while (cur1 <= mid && cur2 <= right) // 降序
        {
            if (nums[cur1] <= nums[cur2]) {
                tmpNums[i] = nums[cur2];
                tmpIndex[i++] = index[cur2++];
            }
            else {
                ret[index[cur1]] +=
                    right - cur2 + 1; // 重点 
                tmpNums[i] = nums[cur1];
                tmpIndex[i++] = index[cur1++];
            }
        }
        // 4. 处理剩下的排序过程
        while (cur1 <= mid) {
            tmpNums[i] = nums[cur1];
            tmpIndex[i++] = index[cur1++];
        }
        while (cur2 <= right) {
            tmpNums[i] = nums[cur2];
            tmpIndex[i++] = index[cur2++];
        }

        for (int j = left; j <= right; j++) {
            nums[j] = tmpNums[j - left];
            index[j] = tmpIndex[j - left];
        }
    }
};